This invention relates to silica supported chromium catalysts containing titanium.
Supported chromium oxide catalysts have been used for many years in the polymerization of olefins to solid polymers. One widely used method of producing such catalysts involves precipitating a silica hydrogel, impregnating the hydrogel with an aqueous solution of a chromium compound and drying. Such procedure gives a silica which inherently has sufficient strength to resist severe shrinkage of the pores during simple drying techniques such as oven drying, tray drying, spray drying or drying under a heat lamp. Such catalyst is simple and inexpensive to produce and gives outstanding results in solution polymerization of olefins to give normally solid polymer.
There is an economic advantage in some instances to producing olefin polymers in a slurry as opposed to a solution system. However, certain control operations which are easily carried out in the solution process are considerably more difficult in the particle-form or slurry process. For instance in the solution process, control of the molecular weight can be effected by changing the temperature, with lower molecular weight (higher melt flow) being obtained at the higher temperature. However, in the slurry process, this technique is inherently limited since any effort to increase the melt flow to any appreciable extent by increasing the temperature causes the polymer to go into solution and thus destroys the slurry process.
It is known that titanium affects the polymerization activity of silica supported chromium catalysts in a way that is of special importance in slurry polymerizations. However, when titanium is coprecipitated with the silica, it produces a hydrogel which does not have sufficient strength to resist serious collapse of the pores during simple drying such as spray drying. Similarly, in accordance with the prior art, if an aqueous solution of a titanium compound is added to the dried silica, the pores are damaged. Accordingly, in order to take full advantage of the improvement which can be imparted to the melt index capability through the use of titanium in accordance with the prior art, the titanium had to be coprecipitated with the silica and the resulting hydrogel (cogel) dried by a more expensive azeotrope distillation or washing with a liquid oxygen-containing water soluble organic compound.